Conduction and breakdown in dielectric liquids

,

t

•

l

Conduetion and breakdown in dieleetric liquids

Conduction and breakdown

in dielectric liquids

Proceedings of the

5th international conference organized by the Department of Applied Physics of the

Delft University of Technology,

Noordwijkerhout, the Netherlands, 28-31 J uly 1975

Edited by J. M. Goldschvartz A. K. Niessen W. Boone Preface by B. S. Blaisse

Delft University Press / 1975

•

I,

I

fi

i

3

~

/

~i:;-')

- ---'-- - ~: -~.~'~. ~j

Organizing commitfee

J. M. Goldschvartz (chairman) Delft University of Technology A. K. Niessen

Philips Research Laboratories, Eindhoven W. Boone

N.V. Kema, Arnhem Advising commillee

1. Adamcsewski / Gdansk, Po land B. S. Blaisse / Delft, the Netherlands J. H. Calderwood / Salford, Great Britain

N. J. Félici / Grenoble, France

E. O. Forster / New Jersey, United States of America eh. Frei I Genève, Switzerland

T. J. Gallagher / Dublin, lreland

Y. lnuishi / O~aka, Japan

T. J. Lewis / Bangor, Great Britain

F. Scaramuzzi

/ Rome,

Italy

Copyright © by Nijgh-Wolters-Noordhoff Universitaire Uitgevers B.v., Rotterdam.

No part of this book may be reproduced in any form, by print, photoprint, microfilm or any other means without written permission from the publisher. ISBN 902980300 2

CONTENTS

PREFACE

MOBILITY AND CONDUCTION

U. Sowada, G. Bakale, K. Yoshino and

W.F. Schmidt/Electron transport in high mobility liquid hydrocarbons and

tetramethylsilane

M.R. Belmont/A maximum bound for electronic

XI

mobilities in liquid hydrocarbons

5

I. Kalinowski, J.G. Rabe and W.F. SChmidt/ Electron drift mobility measurements in liquid 3-methylpentane and the effect of

impurities 11

M.P. de Haas, J.M. Warman and A. Hummel/

Hole conductivity in liquid hydrocarbons 15 K. Yoshino, K. Yamashiro and Y. Inuishi/

Carrier mobility in liquid crystals '

19

~N. Félici, B. Gosse and J.P. Gosse/ Electrical

conduction in nematic liquid crystals 23

HIGH-FIELD CONDUCTION

R.E. James/Conduction adjacent to oil-immersed

dielectric surfaces in a non-uniform field 27 R. Tobazéon and E. Gartner/The effect of solid

polymeric materials on the ionic conductivity of liquids under high electric stress

P.B. McGrath and J.K. Nelson/Light emission studies in the interpretation of high-field

32

conduction of dielectric liquids 36

J.K. Nelson and I. Hashad/Frequency dependence of stress-induced cavitation in fluorocarbon

C.J. Buffam and J.E. Brignell/High-field

conduction pulses in n-hexane

HIGH-FIELD CONDUCTION

W.R.L. Thomas and E.O. Forster/Electrical

conductance and breakdown in liquid

hydro-45

carbons

49

C. Kao/On the theory of filamentary single

injection and high-field electric conduction

and breakdown in dielectric liquids

55

M. Zahn/Transient electric field and space

charge beh

.

aviour for drift dominated bipolar

conduction in dielectric liquids

61

CONDUCTION-IMPURITIES

J.

Evison and

J.E.

BrignelljSimulation of

charge transport following unipolar injection

65

J.M. Warman, M.P. de Haas and A. Hummel/The

effect of trapping and detrapping of electrons

on the negative charge carrier mobility in

liquid tetramethylsilane

70

G.B. Denegri, G.

Molinari

and A. Viviani/

A contribution to the study of low-field

conduction by impurity particles in dielectric

liquids

74

F. Abgrall and J.M.

Cardon/Influence

of solid

impurities on the electric strength

of

transformer oil

B.

Young/High-field

conduction in heavily

contaminated transformer oil

S. Sakamoto and S. Usuda/Effect

of

added

electronegative substances (SF

₆

,

1

_{2 ) on ion}

mobility in mineral oil

INJECTION

S. Barret, F. Gaspard, F. Mondon/

Mo

nitoring of

79

84

90

injection processes in dielectric liquids

94

W. Tauchert and W.F. Schmidt/Charge injection

in dielectric liquids by photoelectric emission

98

N.

F~lici,

B. Gosse and J.P. Gosse/Factors

controlling ion injection by metallic electrodes

in dielectric liquids 103

R. Tobazéon and M. Sauviat/Conduction in insulating liquids due to carrier injection: nature of carriers injected and regeneration

thereof by ionic polymers 107

T.R. Hewish and J.Eo Brignell/Photo-injection

into n-hexane 111

J. Casanovas, R. Grob and D. Blanc/Free ion yields of some dielectric liquids and study of binary mixt~res of those dielectrics

irradiated by bOCo and 137Cs y rays 115

EHD

_ Po Atten/Electrohydronynamic stability of

liquids subjected to unipolar injection 119 J.C. Lacroix and P.Atten/Double injection with

recombination: EHD stability and charge transfer 123 J.C. Lacroix and P. Atten/The influence of

induced motion of unipolar charge transport 127 '----.

~ T. Honda and P. Atten/The electroviscous effect

~ and its explanation 131

N. Félici/Electrolytic conduction and EHD

turbulence 135

H. Vanderschueren and R. Coelho/On the role of ions and dipoles on electroconvective

processes in insulating liquids 139

J.D. Cross and M. Eto/Field distributions in

chlorobiphenyls under direct voltages 143

PRE-BREAKDOWN

K. Piotrowski and R. Eyben/Study of a three electrode cell for the recording of the transient pre-breakdown phenomena in liquid

dielectrics 147

C.W. Smith and J.H. Calderwood/Corona and the

molecular structure of insulating liquids 151 J. Fleszyllski/Behaviour of a water drop in the

pre-breakdown processes in insulating oil 155 H. Yamashita, H. Amano and T. Mori/The effect

BREAKDOWN

K. Arii, K. Hayashi, I. Kitani and Y. Inuishij Breakdown time lag and time of flight and

measurement in liquid dielectrics 163

J.L. Maksiejewski and J.H. Calderwood/

Investigation of time-lag characteristics·for

a triggered sp ark gap in a liquid dielectric 167 H. Kishida, T .• Sato and Y. Toriyama/Cathode

fall like discharge in dielectric liquids 171 B. Bommeli, C. Frei, M. Peter/Memory effect in

dielectric liquids being subjected to

recurring discharges 175

G.K.H. SimcoxjThe assignment of appropriate

dielectric stresses in liquids 179

MECHANISMS IN BREAKDOWN

A. Grinberg and D.M.K. de GrinbergjOn the

mechanisms of dielectric breakdown in liquids 183 H. BerteinjBreakdown of oil using one bare and

one electrode. coated with an insulating film 187 J.E. BrignelljEstimation problems in the

step-function breakdown test

B.R. Mears and J.E. Brignell/Breakdown and infra-brea~down phenomena at low rates of stress

193

application 197

CRYOGENIC LIQUIDS

D.

Goodstein, A. Savoia, F •. Scaramuzzi/Ionic mobility measurement in dielectric liquids by

the method of the full space charge limitation 201 Y. Takahaski and T. Sekino/Corona light and

corona pulse nitrogen 205

H. Mitsui, Y. Toriyama and J.H. Calderwood/ The observation of dielectric breakdown in

liquid nitrogen using discharge figures 209 L.P. O'Gallchobhair and T.J. GallagherjSome

aspects of breakdown in liquid nitrogen 213 L. Centurioni, G. Mollinari and A. Vivianij

I

l

in liquid nitrogen under uniform field

conditions

217

L.

Centurioni, B.

Delfino,

G. Molinari

and

A.

Viviani/Dielectric breakdown of flowing

liquid nitrogen under D.C. and

A.C.

voltage

221

MISCELANEOUS

L. Hellemans/Che

m

ical relaxation of the

non-linear dielectric effect in dipole

equilibria

A.

Ohashi,

T. Teranishi and M. Ueda/Power

factor of silicone liquids

-

P.G.

Arnold and

D.

Michelson/Electro-disintegration of dielectric liquids

J.S. Mirza,

H.S.

Zafar and

C.W. Smith/

The faraday-sumoto effect

N. Shammas,

C.W.

Smith and

J.H.

Calderwood/

Pulse generators with liquid dielectric gaps

225

229

233

236

240

PREFACE

The aim of the 5th international conference on

conduction and breakdown of dielectric liquids

is the

same as that of the preceding conferences, namely

to

have a forum where all experts in this vast field

can

meet, can present their latest results and can

exchange

their ideas. It comes within this framework that

the

subjects of this conference cover the whole field

of

conduction and

breakdown

of polar and

non

polar liquids,

including cryogenic liquids.

In comparison

with

many other international conferences

this is a relatively small one. This fact has

the

great

advantage that all participants can

be

lodged in the

congress centre "De Leeuwenhorst" at

Noordwijkerhout.

The result is th

at

everybody who wants to talk

about

a problem with a colleague can easily contact him

and

make an appointment.

When

the organizing committee,

whose

members

are

J.M. Goldschvartz, chairman,

A.K. Niessen

and

W.Boone

began their task, the first idea was to organize

the

conference in the

Aula

complex of the Delft University

of Technology, since the conference is

sponsored by

the Department

of Applied Physics

of the university.

Then the

participants had

to

be

lodged not only in

Delft but

also

in surrounding towns as the number of

hotel beds in Delft is very limited. Thus, the

decision

to

move

into "De Leeuwenhorst"

was practical

and

wise.

The subjects of the sessions of the conference

are:

mobility

and

conduction

of

charge carriers; high-field

conduction; influence of impurities; injection

of

charge carriers; electrohydrodynamics; pre-breakdown;

breakdown

and

mechanisms

of

breakdown; cryogenic

liquids

and

of course also miscellaneous subjects

with

could not be covered by one of the

above

mentioned

titles but are nevertheless important for the

conference.

The contributed papers are printed in this Proceedings,

which is

gjven

to all participants

before

the beginning

of the conference. This was only possible because

the

organizing

committee got

through an

enormous amount of

work and the Delft

University

Press gave its full

cooperation.

The quality and number of the papers ensures the

success of the conference.

Moreover,

I hope the

participants

will come in touch with each other thus

having a fruitful

and enjoyable

time.

•

ELECTRON TRANSPOR T IN HIGH MOBILITY LIQUID HYDRO-CARBONS AND TETRAMETHYLSILANE

U. Sowada, G. Bakale +, K. Yoshino~'~' and W. F. Schmidt*

Introduction

Excess electron mobilities in liquid hydrocarbons exhibit a large spectrum of values ranging from 70 cm 2V- 1s -1 for neopentane to 0.1 cm 2V- 1s -1 for n-hexane at room temperature or from 400 cm 2V-1s- 1 in liquid methane to 10-3 cm 2V-1s-1 in liquid ethane at T

=

l11 oK. In liquids with a low electron mobility the motion is thermally activated and at higher field strengths the mobility increases with field strength. The localized electron model has been applied to interpret these results [1,2). Liquids exhibiting a large electron mobility (> 10 cm 2V-1s-1) show a small temper-ature dependence of the mobility and a decrease of the mobility at higher field strengths . A model of quasifree electrons or band conduction seems to be applicable. Here we wish to discuss the relevant theoretical models and present some new data on tetra-methylsilane.

Experimental Data

The mobility measurement technique, the electrical circuit and the parallel plate cells have been previously des cribed [3). At low field strengths the drift velo city VU increases proportionally to the electric field strength F while above a certain critical field F c the drift velo city increases approximately as FO.5. In figure 1 the results obtained in methane, neopentane, and tetra-methylsilane are displayed. For comparison also data of liquid argon, krypton, and xenon. are shown.

The critical velocity (vc

= j

u(o)._{F c) is comparable to the velo city} of sound in the liquefied rare gases, which indicates the preva:" lence of elastic energy losses. In the molecular liquids the cri-tical velo city is much greater than the velocity of sound. Here inelastic energy losses seemio be the determining factor .

I ';'Hahn-Meitner-Institut für Kernforschung Berlin GmbH, Bereich Strahlenchemie, 1 Berlin 39, Germany

+Now: Case Western Reserve University, ClevelandjOhio, USA

;~*On Ie ave from Osaka University, Dpt.Electr. Engineering, Osaka, Japan

f

....

I(/)

E

u >.

-

_g

10

5 Qj

>

--

"i: ""0

10

4

10

Xenon _-_" -... _-...-

---

.

,,'

,

"

,

.

"

"

"

/'1/

Krypton// Argon / -.../ / Tetramethylsilane

~

field strength [V cm-

1 ]

~

Fig. 1: Electron drift velo city as a function of the field strength.

Models

Argon: tl5 K [4]; Xenon: 163 K [4]; Methane: 111 K [4]; Neopentane: 296 K [5]; 'fetramethylsilane: 195 K, 296 K, this work. Krypton: 121 K,[4] and this work.

In the classical description of electron motion through a system of scatterers the observed drift velocity is the result of acceler-ation by the electric field and energy losses due to elastic or in-elastic collisions . The mobility is given by

f

=

~. .m 't. (1)

where 't is a time which depends on the electron energy and the scattering mechanism. At low field strengths the electrons will be in thermal equilibrium with the liquid and 't' =

I\.

1/ Vth, where Al is the mean free path between collisions and Vth the mean ther-mal velo city . Taking into account the Maxwell velo city distribut-ion gi yes for the mobility

2 2 1/2

r="3(1tmk,T) _{eÀ. 1} (2)

At higher field strengths the energy gain from the field will in-crease the mean electron energy above thermal energies. Equi-librium is obtained if

~

1 I

(d<E» =0

~ t I l_{co lSlons} ' . (3) A more detailed investigation on the motion of excess e1ectrons in liquefied rare gases or po1yatomic liquids was presented by Cohen and Lekner [6], and Davis et al. [7], respectively. They took into account the modification of the scattering process by the liquid structure.

The limit F c of the low field region is re1ated to energy loss by

(4)

Here f is the fractional energy loss per collision (for elastic scattering f

=

~) and S(O)

=

ho , the structure factor for

ther-mal energies. hl

Ao is given by

A -1 2 Il. =4'1tan

o (5)

with a the scattering length and n the number density of atoms or molecules. In principle, fis determined by eq. 4 if a is known. The scattering length a can be obtained from the value of Vo' the energy of the conduction state by the semiempirical method published by Fueki et al. [8]. These calculations have been done for the liquids of fig. land the results are summariz-ed in table .1. Table 1 Experimental Data Calculated

fo~

y+ *~

Id

A§

Liquid T[K] F ~ _stol 0 c 0 Ar ~5 -0.2 0.3 154 4.3 0.02tl Kr 121 700 (-0.7~; O,I~ 320 4.76 0.015 Xe 163 1900 -D.65 0.05 1000 3.6 0.0036 Me 111 400 -0,15 1,5 175 2.7 0.015 NP 296 70 -0,43 20 50 3.22 0,064 TMSi 296 100 -0.6 20 72 1.83 0.025 ó' 2 -1 -1 + ** -1 t: Ci) [cm V s ]; [eV]: [kV cm ]; II [1\.]

Vo-valuesfrom W.Tauchert, this conference and [9] 1) estimated value by Raz and J ortner Values f _{fel a -5} f/fela .10- 5 3.7 2,7.10 1,37 1.5 .10-5 1 3.10- 5 1,15 0.15'10- 5 0'8,10- 5 _0,2 40 .10-5 7' .10- 5 5,7 330 .10-5 1,5'10-5 220 274 ' 10-5 1,2'10- 5 230

Although these calculations have onlyapproximate character, the different behavior of excess electrons in atomic and polyatomic liquids is clearly demonstrated. While in the liquefied rare gases elastic losses account for the observed mobility and the critical field F c in polyatomic liquids the fractional energy loss f is much greater and transfer of energy to internal vibrations has to be considered. This would explain the similar behavior of neo-pentane and tetramethylsilane.

Acknowledgement: We thank "Deutsche Forschungsgemeinschaftll for financial support.

References

[1] W.F.Schmidt, G.Bakale, and U.Sowada, J.Chem.Phys.61, 5275 (1974); [2] G. Bakale, U .Sowada, and W.F. Schmidt, 1974 Ann.Rep. Conf.Electr.lnsul.Dielectr.Phenom.; [3] G. Bakale and W.F. Schmidt, Z. Naturforsch. 28a, 511 (1973); [4] L. S.Mil-Ier, S.Howe, and W.E.Spear, Phys.Rev. 166, 871 (1968); [5] G. Bakale and W.F .Schmidt, Chem.Phys. Lett. 22, 164 (1973); [6] M.H.Cohen and J . Lekner, Phys.Rev. 158, 305 (1967); [7] H.T.Davis, L.D.Schmidt, and R.M.Minday, Phys.Rev. A3, 1077 (1971); [tlj K.Fueki, D.F .Feng and L.Kevan, Chem-:Phys. Lett. 13, 413 (1972); [9] R.A.Holroyd and M.Allen, J .Chem. Phys. 54, 5014 (1971)

A MAXIMUM BOUND FOR ELECTRONIC MOBILITIES IN LIQUID HYDROCARBONS

M.R. Belmont·

Introduotion

Caloulations have been made [1 .... 3J of the delooalised or quasi-free eleotron mobility in highly purified liquid hydrooarbons. For the evaluation of the distribution funotion and mobili ty the Cohen Lekner [ 4] analysis of plane wave scattering was used, the resulting expression for

yo

is

2( 2

)i_e_

}Jo'"

3~;'

m kT n S(o)CJ.

(1 )

where n is the number density of the liquid moleoules, k is Boltzmann's constant, T the absolute temperature, m the

eleotron rest mass, e the electronio oharge, cl the scat tering cross section, 8(0) is related to the pair correlation

function of the liquid and brings in the allowed phonon exoitations of the medium. It's value is given by 8(o) ... nkTK~

where KT is the isothermal oompressibility of the liquide Reasonable agreement was obtained with experimental results

/ for the spherical isotropio moleoules of neopentane [5, 6, 9] and methane [7

J.

However the technique failed when applied to the results for the linear molecule n-hexane [5,8J. A weakly temperature dependent drift mobility was predicted, having the value of 1.7 x 10-2. m'l. v-I sec-I [1J or 1.5 x 10-Z. m:1 V-I sec-I [ 3] at 300ok. This was in strong contrast to the experimentally observed mobility V [5,8] which has a magnitude of approximately 10-~ m2 _{v-I sec~ at 300}0_{k and exhibits an} ex~onential dependenoe on temperature as given by equation

(2)

(2)

A is essentially temperature independent compared with the exponential term, and ~ is the activation energy. It was noticed [1~3J that by empirioally equating the oalculated value for'~o with A in equation (2) and using the experimental value for

Y')

the correct value of}J could be obtained. The authors [1~3] olaimed that this procedure should be valid for the other n-alkanes as weIl as n-hexane. Unfortunately

activation energies are not available for other normal paraffins to ensure that this approach does not represent merely an isolated coinoidental agreement.

Capture Cross Seotion

One feels that the agreement for n-hexane may be purely fortuitous for the following reasons. Firstly, most authors

"Glamorgan Polytechnic, Llantwitroad, Treforst, Pontypridd,

in this field assume that the form of the observed temperature dependenoe of strongly indioates that it is oontrolled by soma thermaly aotivated release prooess involving looalised levels. This may be a) quasi-free eleotron inter-trap motion via an energy band or b) hopping i.e. aotivated tunnelling between sites as disoussed by Mott [ 10

J ,

Davis and Mott [ 11] MilIer and Abraham[12] and others. Both olasses of prooess, prediot the temperature dependenoe given by equation (2) but A would not be solely determined by

po

in either oase.

Seoondly if one assumes prooess a) whioh is favoured by many workers [1+3,

5->8, 13]

then equating}Jo wi th A requires that the produot

v't

o is uni ty .Here lko is the a ttempt tb esoape frequency and

V

is the trapping frequenoy • lito has been

reasonably assumed to correspond to phonon frequenoies [ 3] i.e. l:o""'10-1~ sec. This then requires that V"'10 1'5 seo-I • Suoh a

high oapture rate requires unreasonably large values of aotive oapture oross seotion for the proposed neutral liquid traps. This is beoause the dipole moment required to quarantee looalisation of an eleotron is aurpriaingly large [21).The chanoe of a single moleoule exhi-bi ting suoh a moment as a result of transient thermal distortion destroying the cancel-lation of group moments or of several molecules simultaneously aquiring the required moment to produce the same overall effect is very smalle Thus the trap density must be low hence

b would need to be very large. Finally it ia also interesting to note that the distanoe to oapture is equal to or leas

than the momentum mean free path valuea quoted for

yo •

It seems from the above reasoning one must oonolude thatV«1013 seo-l , with the result that any quasi-free electron mobility for n-hexane must be much lower than the calculated figure of approximately 1.5 x 10-Zm2.V-l -sec-I_[1,3].

Maximum bound on Y-o

If one aooepts for the moment that delooalised eleotrons oan exist in n-alkanes then the following simple oaloulation sets an ~pper limit tO}Jo of 1. 4 x 10-3 _{m 2. v -I sec -I in n-hexane at} 300 k. Sohmidt and Allen [5J found that the drift mobilitY6 was independent of applied field strengthe up to 8.3 x 10 V.m-l _{in n-hexane and 1.4 x 10}7 _{V.m -I in pentane [ 5}

_J.

_Then

unless some variation in the trap parameters is exactly oan-oelling a variation inp , whioh seems improbable, one oan _

assumeyois stress independent in the two alkanes up to these field strenghts. There will be soma stress at whioh}Jo beoomes stress dependent as the mèan value of the drift oomponentVd of -the eleotrons velooity beoomes oomparabIe with the thermal oomponentVc • The absenoe of stress dependenoe in the results of Sohmidt and Allen means that

Vol

is small oompared to Ve up to the maximum field strengths used. Bearing in mind their order gf experimental error one oan say with oonfidenoe that at 300 k

Vd - ~ 0.1 Vo

Now by def'ini tion Vil = )Jo~ and Vc can be obtained when inequal-ity (}) holds f'rom the expression

3

1

*

2

- kT = - m V

(4)

2 2 c

where m* is the electron's ef'f'ective mass. Equation (4) assumes the classical limit of' the Fermi Distribution which is certainly true f'or the very dilute excess electron popula-tions in the insulating liquids considered.

To establish a minimum value for m* one first notes that the molecular overlap in liquid n-paraffins is weak. Thus the resulting energy bands will be narrow. The electron's wave vector thus varies slowly with energy and consequently m~ is large. Hencé m* is unlikely to be smaller than in a narrow band single crystal organic sOlid, so one can reasonable write

near the bottom edge of any quasi free electron band in n-alkanes. In

(5)

m is the electronic rest mass.

Substi tuting for Vd, and V_c into inequali ty (}) and using the maximum stress reached by Schmidt and Allen gives an upper bounds for~oof 1.4x10~ m~.v-( .sec-I in n-hexane and 8.2 x10-4 m2 .V-I.sec-I., in n-pentane, both at }OOok.

The method used is obviously approximate as the in-equalities (2) and

(5)

are rather flexible. However as maximum allowable values have been used the upper bound on}Jo is too high rather than too low and thus represent the very largest possible value of "):!O which is one tenth the valué. calculated for n-hexane [ 1~}] •

Alternative Model

With such a low maximum mobility value for supposedly highly delocalised electrons one is forced to re-examine the use of a

quasi free scattered plane wav", approach to the problem [ 1-+3].

Ex~ctly the same reappraisal was required for amorphous solids

when workers in that field tried to fit conventional crystal-line band models to their results. The situation there was resolved by the randomly perturbed periodic potential theories which lead to the current models [11,14~16] for electron energies in the amorphous phase. These same models will now be considered as alternative descriptions for conduction in n-alkanes. The presentation due to Davis and Mott [11] and Mott

15

will form the basis of the discussion, although this treatment is not wholey quantative it gives a good physical insight into the problem. In any case the spread in reported experimental data, particularly the activation energies [5, 81 in n-hexane is too large to justif'y detailed quantative

treatment.

The basic model is of an increasingly more diffuse extension of the energy band beyond the conventional band edge for crystalline solids, as shown in figure 1a.

/

Conventional Baud ._~ap~ tail Statee

Localieed States

__ ~~~~lieed Statee

Band Tail Statee ...

_----Conventional Band Density of Btatea

Fig. 1a

Energy ___ B.a_n~. 'Mobili ty __ .MppHi ty Shoulder Localieed Conduction ___ ~o_cali . . d Conduction _ -'~_o!>gi ty Shoulder Band Mobili ty Mobili ty }J

Fig. 1b

As the density of states falls away from the band edge so

does the inter-site tunnelling probability. Thus Mott suggests

that within the regions

Ee~Ea

and

Ey+Eb

the conduction is

atill delocalised in character, however the effective mass

rises very rapidly as one moves out into the gap region. The

consequences are the mobility shoulders Fig. 1b, where the

delocalised mobility falls by orders of magnitude. Outside

Ea

and

Eb

delocalisation is supposedly no l0nger possible,

charge

~tion

is then via thermally activated inter-site

tunnelling (activated hopping). For a wide band insulator the

Fermi level will be far removed from the band edges and the

diffuse or 'tail states' provided a very large density of

carriers is not injected. Now even at very high field strengths

few carriers gain energies above a few tenths of an electron

volt. Thus the majority of charge transport will occur in the

'tail states' and not the band proper. Davis and Mott [11]

suggested that for electrons in calcenogide glasses yoin the

region

E(~Ea

is of the order of

10-3

m2..V-'.sec-

l ,

at

300

o

k.

It is interesting to no te

.

that this is the order of magnitude

of predicted by inequality

(3).

In terms of this model the experimental temperature

depen-denoe ofy' would be explained in two ways:

a) At moaerate and high temperature by a capture and release

meohanism fr om localised sites lying between

Ea

and

Eb

with

delocalised inter-site motion via the mObility

,

shoulders,

this is described by equation

(6)

(6)

}!t

is the mobili ty wi th in region

Ec~ Ea , E

the effeotive si te

depth below

E,

and

~

is dependent on the oapture parameters

and the distribution in energy of looalised states.

aotivated tunnelling, without delooalised eleotron motion between sites. The mobility)U is then given by

Due to the low tunnelling probabili ty below E éiI the

inter-site transfer term R in equation (1) is very muoh less than

yt~

.

The hopping aotivation energy W, is of ten very low, typically 0,01 eV [ 17

J

for amorphous aluminium oxide, silicon oxides and silicon nitride. It has been suggested

[15J

that for hopping in liquids the continuously changing atomie or molecular configuration removes the requirement for an acti-vation energy i.e. W=O. The order of magnitude of}J is then roughly that for ionic mobility in liquids.

The magnitude of the ac ti va tion energy 0.18 ev -,> 0.2 ev [5, 8 ]

and the absolute value ofy in n-hexane tends to indicate equation

(6)

behaviour. Furthermore the change over to hopping at low temperatures of ten observed in amorphous solids does not seem to be present in liquid n-hexane for the activatàon energy remains constant down to the freezing point at 200 k. Local Anistropy

Having suggested the proposed mechanism for mobility in linear alkanes one might ask why spherical molecular liquids like tetramethylsilane and methane do not exhibit similar behaviour instead of the highly delocalised conduction which experimen-tal resul ts point to [

5, 7].

The dilemma is resolved by recognising that localised states and hence the band exten-sions or 'tail states' leading to them, only appear in a disordered system when the product of a potential fluctuation and the square of its effective radius are above a minimum value. Gubanov [20] shows that this minimum is lowered i f

there is any anistropy in the medium. For systems exhibiting any degree of chain or layer structure the density of

local-~sed levels may be many orders of magnitude larger than for wholey isotropie materiaIs. This argument has been used to explain why 'tail states' are found in amorphous chain or layered semiconductors [21-+23J while there is some doubt about their existance in isotropie amorphous silicon and germainium [24, 25]. Vapour pressure measurements 26 indi-cate that linear alkanes exhibit substantial rotational hin-de rance, the molecules packing locally along the chain axis. Thus an excess electron would 'see' a locally anistropie environment. The density of localised sites below Ec is thus much larger for linear alkanes than for the spherical molec-ular liquids. Rence the alkanes would be most likely to exhibit a trap controlled mobility, while the spherical mole-cules would exhibit quasi free electron transport. The low value for~o in n-hexane predicted by inequality

(3)

would fit in with the very high effective mass values expected in the

'tail states' between

Et

and Ea.

The anistropy argument also prediets the variation in observed in mixtures of n-hexane and neopentane, for as the concentration of hexane falls so does the degree of local anistropy.

References

1. K. Fueki, D.F. Ferg and L. Kevan, Chem.Phys.Lett. 13, 616,

1972

2. K. Fueki, Canad.J.Chem. 50, 3370, 1972

3.

H.T. Davis, L.D. Schmidt and K.M. Minday, Chem.Phys. Lett.

13, 413, 1912

4. M.H. Cohen and J. Lekner, Phys.Rev. 158, 305 1961

J. Lekner, Phys.Rev. 158, 130, 1967

5. W.F. Schmidt and A.O. Allen, J.Chem.Phys. 52, 4788, 1970

6. R.M. Minday, L.D. Schmidt and H.T. Davis, J.Phys.Chem. 76,

442, 1912

7. W.F. Schmidt and G. Bakale, J.Chem.Phys. 11, 611, 1912

8. R.M. Minday, L.D. Schmidt and H.T. Davis, J.Chem.Phys.,

1911

9. J.P. Dodelet and G.R. Freeman, Canad.J.Chem. 50, 2667,

1972

10. N.F. Mott, J.Non-Cryst.Sol. 1, 17, 1967

11. E.A. Davis and N.F. Mott, Phil.Mag. 22, 903, 1970

12. A. MilIer and E.Abrahams, Phys.Rev. 120, 745, 1960

13. Mo.Cubbin and Gurrey, J.Chem.Phys. 43, 983, 1965

14. M.H. Cohen, H. Fritzsche and S.R. Ovshinsky, Phys.Rev.

Lett. 22, 1065, 1969

15. N.F. Mott, Phil.Mag. 24, 1, 1911

16. A.I. Gubanov, 'Quantum Electron Theory of Amorphous

Conductors', Consultants Bureau N.Y., 1965

17. A.K. Jonscher, Thin Solid Films, 1, 213, 1967

18. P.G. Le Comber and W.E. Spear, Phys.Rev.Lett. 25, 509,

1970

19. E.L. Rossiter and G. Warfield, Dept. of Elect. Engg.

Device, Phys.Lab., Prinoeton Univ.Tech.Rep. no. 10, 1961

20. A.I. Gubanov, Sov.Phys.-Semicord, 6, 1202, 1973

21. H.P.D. Lanyon, Phys.Rev. 130, 134. 1963

22. B.T. Kolomiets et.al., J.Non-Cryst.Sol. 4, 45, 1970

23. A.E. Owen and J.M. Robertson, J.Non-Cryst.Sol. 2, 40,

1970

24. T.M. Donovan et.al., Phys.Rev.Lett. 22, 1058, 1969

25. D.T. Pierce and W.E. Spicer, Phys.Rev.Lett. 27, 1217, 1971

26. L.H. Thomas, J.Chem.Soc. (A), 2609, 1968

ELECTRON DRIFT MOBILITY MEASUREMENTS IN LIQUID 3-METHYLPENTANE AND THE EFFECT OF IMPURITIES

I.

Kalinowski, J.G.Rabe, and W.F.Schmidt'~

Introdu ction

For the elucidation of the electron transport process in disorder-ed systems measurements of the mobility of excess electrons in many systems have been performed. Several methods for the de-termination of the drift velo city of excess charges in an electric field are available [e. g. 1], Bakale and Schmidt used a burst of high energy X-rays to produce a homogeneous distribution of charge carriers of either sign between the parallel plates of an ionization chamber [2].

Under the influence of an electric field the charge carriers drift towards their respective electrodes giving rise to a current which de creases in time. In liquid hydrocarbons a fast and a slow de-cay have been observed which were attributed to the drift of elec-trons and positive ions. Sometimes the purity of the liquid is not sufficient and electron attachment to impurities occurs. The por-tion of the current which is carried by the electrons does not de-crease linearly with time but the apparent time constant of the decay may be still influenced by the strength of the electric field, It is the purpose of this paper to show how drift mobilities can be obtained in these cases .

. Theoretical Considerations

After a short burst of high energy X-rays is applied to the liquid a uniform density n+ of positive and negative charge carriers exists between the - plates of the measurement cello With an ap-plied voltage V an ionization current i(t) is measured which is

given by V

i(t)=nt(t) eo(ju.. +

r+)

.

d'

q (1) where e means the electronic charge, u+ the mobility of the positiveoor negative carrier,

respectivJy~

d the electrode sepa-ration and q the electrode area. In the case of electronic conduct-ion

r e l » r+ so that eq. 1 reduces to

Hahn-Meitner-Institut für Kernforschung Berlin GmbH, Be-reich Strahlenchemie, 1 Berlin 39, Germany

(2) Electrons may be lost for the conduction process due to three different processes:

1. Excess electrons are neutralized on the anode

2. Volume recombination between electrons and positive ions occurs

3. Electrons are scavenged by impurities and the mobility of the negative ion is much smaller than the electron mobility. In case 1 the current decreases linearly in time as

ti

V

iel(t)= iel(o) (1 -

I

;i .

t) (3) The decrease of n 1(t) due to the processes 2 and 3 is described

by e

d nel

--;:u-

= - k_{1 nel n+ - k 2 nel ns} (4)

where k

1 and k2 are the rate constants for recombination and at-tachment, respectively, ns is the concentration of scavengers. The relatïve importance ofthe two processes can be estimated for the conditions of our experiments. It turned out that k

1 n 1 n

+

was always several orders of magnitude smaller than K2 ~el ns and, therefore, recombination was neglected. Equation 4 be-comes

which has the solution

-k n t 2 s

nel(t) = n(o) el e

(5)

(6) The electron current in the presence of scavenging is then given by

(1 - t ) (7)

If the drift time d2

I

jU IV is comparable to 11k n then the de-cay will not be linea!r e but will still dep end on". \lreasurements at increasing voltages V give decay curves iel(t) with decreas-ing decay times. The constants k2ns and rel are determined bya curve fitting procedure. We choose a value of IUel and plot

i l(t)

IU

IV

In

~()

(1 -

~2

t) versus t for the different

volta-lel 0 d

With the correct rel a set of parallel straight lines is obtained, from which k 2 ns can be determined.

Experimental

The electrical circuit and the cell have been previously describ-ed[2]. Electrode distances varied between 0.1 and 0.05 cm. Puri-fication of the 3-methylpentane was effected by chromatography through columns of activated silica gel, preirradiation under high vacuum with 60Co-î'-rays and further degassing by trap to trap distillation in a high vacuum. The measurements we re perform-ed in the temperature range 258 K to 371 K and with electric field strengths up to 250 kV cm -1.

Results and Discussion

The decay of the electronic component of the ionization current was recorded oscillographically and in figure 1 typical data ob-tained in liquid 3-methylpentane at 430C are shown. The solid lines represent the dependence according to eq. 7 with a mobili-ty of

~el

=

0.34 cm 2V- 1s- 1 and k2ns

=

2.5 x 10 5 s-l. The va-lues tor k2 ns varied between 10 5 and 6 x 10 5 for different pre-parations . Scavenging rate constants have been measured for se-veral solutes in a number of solvents. Values up to 10 14 I mole- 1 s-l have been reported [e.g.3]. Taking 2.5 x 10 12 I mOle- 1s- 1 (which may be realistic for our experiments) we estimate the or-der of magnitude of the residual impurity concentration to 10- 7 mole/1 or 6 x 10 13 cm -3. This figure should be compared to the

Fig. 1.0 . . - - - ,

t

ë 0.5 ;::;

-;::; , , , , , _{, ,} , , , , _, , , _, , , _, , _, , ,,~~

,

" al " bi ... - '-,

o

L----'-_---i-~..l!!.._~I:::.._____"_~___'

o

2 3 4 5 6 time [~secl ~ 1: Decay of the electron current in 3-methylpentane at t = 430C a) 57.1 kV cm- 1; b) 66.7 kV cm- 1; c) 76.2 kV cm- 1; d) 85.7 kV cm- 1; e) 95.2 kV cm- 1,:

2.0

;(/)

lO

>-N E u

g

15 0 E c: _0.1 0

...

·ti ~ Qj 0.03 350 2.6 temperature [Kl 3.0 300 3.5 3

.1!L

[K-'l T 250 4.0

Fig. 2: Temperature dep enden ce of the electron mobility in 3-methylpentane

number of 3-methyl pentane molecules 4 x 10 21 cm- 3 • A very small concentration of electron attaching impurity is sufficient to cause deviations from the linear current decay or in other words the liquid has to be purified to such an extent so that less

than 1 scavenger molecules remains in 108 solvent molecules.

The temperature dependence of the mobility is shown as an Arr-henius plot in figure 2. The activation energy was determined to

Ea

=

0.2 eV.A trapping model seems to be applicable. Some

evi-dence for field dependent mobilities above 200 kV cm- 1 was

ob-tained.

References. [1] A.Hummel and W.F.Schmidt, Radiat.Res.Rev.

5, 199 (1974); [2] G.Bakale and W.F.Schmidt, Z.Naturforsch. 28a, 511 (1973); [3] W. F. Schmidt, HMl-Report B-156 (1974).

HOLE CONDUCTIVITY IN LIQUID HYDROCARBONS

*

M. P. de Haas,

J.

M. Warman and A. Hummel

Initial indications that the positive ion ("hole") formed on ionization of liquid saturated hydrocarbons by high energy rad ia-tion might have a mobility significantly larger than that expected for a molecular ion came from steady-state and optical pulse radiolysis studies of cyclohexane solutions. The formation of a high mobility hole in cyclohexane was recently confirmed by re-sults obtained1 using a microwave absorption technique to measure the radiation induced conductivity in this liquid on a nanosecond timescale. We report here a brief account of further studies of the radiation induced conductivity in liquid hydrocarbons.

The basis of the present method of studying ion formation in pulse irradiated liquid hydrocarbons is the attenuation of micro-waves which results when propagation occurs in a conducting medium. Briefly, microwaves are incident upon a cell, containing

the liquid of interest, which consists of a length of X band waveguide closed at one end by a glass transmission window and at

the other by a short circuiting metal plate. The level of micro-wave power reflected from the cell is monitored as a function of time immediately following irradiation of the liquid with pulses of 3 MeV electrons from a Van de Graaff accelerator.

For absorptions less than 5% the fractional change in reflect-ed power is directly proportional to the conductivity 0 of the liquid and for the present conditions is given by

t:,P/PO 570 (I )

for 0 in units of

n-

1m-1, at the frequency of maximum absorption

(8.45 to 8.60 GHz depending upon the dielectric constant of the liquid) •

The conductivity is given by

o

= Nue

(2)

where N is the number density of ion pairs, u is the sum of the mobilities of the positive and negative ions and e is the elec-tronic charge. If no decay of the ions occurs during the pulse then N is given by GDQd/IOO where G is the yield of ion pairs per 100 eV absorbed, D is the ab~orbed dose in eV per gram per nano-coulomb (average value 1.35 - 0.15 x 1015 in the present experi-ments), Q is the integrated beam current in nanocoulomb (measured by deflection of the beam onto a coaxial target connected to an electrometer) and d is the density of the medium in gram/cm3_• The product of the yield of ion pairs and the mobility (in units of cm2V-1s-1

) may then be obtained from

*Interuniversitair Reactor Instituut, Mekelweg IS, Delft, The Netherlands

Table I Liquid CH MCH IQ Temperature

°c

22 41 61 83 22 22 Gfi (100 eV)-1 0.164 0.204 0.244 0.294 0.125 0.366 u(_)a (cm2V -I s -1) _0.222 _0.304 _0.404 _0.544 _0.0687 _5.42 u(-) ( 11 11 ) 0.20 0.25 0.36 0.45 0.066 6.4 u(+)xI03( 11 11 ) 15 16 18 13 5.8 1.0 u(S±)x 103(

_{" "}

) 0.8 0.8 1.0 1.0 1.1 1.1 Ta) values of reference 4 norma11zed to 0.22 at 22oC.

Gu (3)

Because of the large range of Gu values measured,the same pulse width could not be used throughout but was chosen, within the available range of 2.5 to 250 ns, to be as short as possible while still resulting in a readily measureable absorption signal with a lifetime considerably longer than the pulse width.

The liquids cyclohexane (CH) , methylcyclohexane (MCH) and iso-octane (10) were purified by distillation on a 90 theoretical plate column, deaerated by bubbling with dry N

2, evacuated on a vacuum line and allowed to stand over sodium-potassium alloy for several days before use.

From the fractional absorption of microwave power following pulse irradiation of pure CH, MCH and 10 at 220C values of Gu

=

C[u(-) + u(+~ , where u(-) and u(+) are the electron and hole mobilities, of 3.5 x 10-2, 8.6 X 10-3 and 2.30 cm2V- 1s- 1(100eV)-1

respectively were obtained. If geminate ion pairs do not contri-bute significantly to the total yield of ions on the timescale of the present observations and, if the major charge carrier is the electron as indicated by the results which follow, then the above values of Cu should be equal to G

f. u(-) where Gf. is the yield of free ions. Values of Cf' and ut-) determined

È

y other workers are listed in Table I. The1values of Cf' u(-) of 3.5 x 10-2,

8.2 X 10-3 and 1.95 cm2V- 1s- 1 (100eV)-1 1 calculated from the

literature data do in fact agree quite weIl with the Gu va lues obtained for the pure liquids.

The rate constants for reaction of excess electrons with SF 6 i~ the liqui~s studied have been found 2 to.be greater than

10 2mol.dm-3_{s 1 (the rate constant for MCH 1S presumed to be}

approximately equal to that found in n-hexane, a li~uid with a similar electron mobility). In the presence of 10- M SF

6 the electron lifetime will therefore be reduced to less than 0.1 ns and, if the electron is the major charge carrier the end-of-pulse value of Gu,

=

G[u(+) + u(S-)] where u(S-) is the mobility of the molecular ion resulting from electron capture by SF

6 ' will be considerably lower than in the pure liquid. A decrease by a factor of approximately ten in Gu on addition of SF

6 to CH has been previously observed 1and a similar decrease was found in the

Figure I. The product of the ion pair yield, G (IOOeV)-l, and the sum of the ion mobilities, u[cm2V-1s-J' as a function of time following pulse irradi-ation of MCH containing: no additives, .; 10-2 mol/dm3 SF6~ +; and 10-2 mol/dm SF₆ + 5 x 10-3 mol/dm3 NH3' O. 10-4~ ______ ~ ______ ~~

o

50 100 t/ns

present study for MCH, Figure I. In 10 an even more pronounced decrease in Gu, by a factor of 3 x 10 3, was observed on addition of SF

6• The electron is therefore confirmed to be the major charge carrier in the three liquids studied.

In CH it has been found1 that a further substantial decrease in Gu occurs on addition of NH₃ to a solution containing SF 6.~.'

This has been explained by the reaction of holes with NH

3 to form

positive ions, S+, of a much lower mobility. The value of Gu in the presence of both NH and SF

6 is then given by G[u(S+) + u(S-~ A further reduction in

Cu

is also found to occur on addition of 5 x 10-3M NH

3 to a solution of SF6 in MCH, Figure I. For 10 how-ever, Gu remains almost unchanged on addition of NH

3.

From the values of Gu determined in the pure l~quid and in

~olutions containing SFn and SF

6 + NH3, values of u(-), u(+) and

u(S±)

=

0.5 [u(S+) + U(S-)] may be derived if it is assumed, as a first approximation, that G

=

G_f .• The values obtained for CH over the temperature range 22 to

83

0_{C and for MCH and 10 at 22}0_C

are listed in Table I.

The values of u(-) obtained in the present investigation are in reasonably good agreement with eleètron mobilities determined by other workers. While the values of ~(S±) found are of the magnitude expected,they are possibly somewhat too high since it

is known that for molecular ions a significant yield of geminate ion pairs may be present on a nanosecond timescale.

The hole mobilities in both CH and MCH are seen to be consi-derably larger than for molecular ions in the same liquid, where-as for 10 u(+) is approximately equal to u(S±). These findings are qualitatively in agreement with the conclusions reached on the basis of the results of an optical pulse radiolysis study3 of the kinetics of charge transfer to solute molecules in the present liquids.

In contrast with the more than twofold increase in u(-) in cyclohexane in going from 22 to 830C, u(+) is seen to be almost independent of temperature.

A possible explanation of rapid hole transport in CH and MCH may be resonant charge transfer between positive ions and neigh-boring molecules. This would require the geometrical config ura-tions of the ion and ground state molecule to be closely similar, a condition which is possibly not fulfilled for 10. Clearly more

experimental data are required however, before a full understand

-ing of the nature of hole transport in liquid saturated hydro-carbons is possible.

References

I) M. P. de Haas, J. M. Warman, P. P. Infelta and A. Hummel, Chem. Phys. Letters, in press.

2) A. O. Allen, T. E. Gangwer and R. A. Holroyd, J. Phys. Chem.

?.:i,

25 (1975).

3) E. Zador, J. M. Warman, A. Hummel, Chem. Phys. Letters ~, 363 (1973).

4) J. P. Dodelet and G. R. Freeman, Can. J. Chem. 50, 2667 (1972). 5) J. H. Baxendale, C. Bell and P. Wardman, J. Chem. Soc. , Farad.

Trans. I, ~, 776 (1973).

6) W. F. Schmidt and A. O. Allen, J. Chem. Phys.

g,

2345 (1970). 7) A. O. Allen and R. A. Holroyd, J. Phys. Chem.-78, 796 (1974).

CARRIER MOBILITY IN LIQUID CRYSTALS

K. Yoshino! K. Yamashiro*and Y. Inuishi*

Introduction

Liquid crystals is becomming more important as an element of the

display device. However the detail of the mechanism of the

ele-ctrical conduction which has close relation both with the light scat tering and the memory effect are not well understood. In this paper we present the mobility measurements in various liquid crystals at various phases (isotropic liquid, nematic, sme-ctic and cholesteric states) as functions of the temperature, the

applied electric and magnetic fields. Effect of surface condition

of the electrodes will be also mentioned. Experimental

Several liquid crystals [ cholesteric type: cholesteryl nonanoate (CN), cholesteryl propionate (CPr) , cholesteryl palmitate (CPa) etc.; smectic type: n-p-cyanobenzylidene-p-n-octylaniline (CBOA), ethyl p-methoxy benzylidene amino cinnamate (EMBAC) etc.; nematic type: p-azoxyanisole (PAA) etc.] were obtained from Fuji Dye Co. Ltd.and were purified by recrystalization for several times from

appropria,te solution. The samples were sandwiched between two

nesa coated or gold evaporated glass electrodes and all

measure-ments were done in argon (99.99%) atmosphere. Mobility was

measur-ed by the methods of transient SCLC, polarity inversion of tqe

applied field or radiation induced condition. The transmission

lightof He-Ne laser was monitored by a photomultiplier lP2l

simul-taneously. The magnetic field up to 15 kG was applied

perpendi-cularly or parallel to the electric field if necessary. Results and Discussion

1. Smectic and Nematic Liquid Crystals

Figure 1 shows the temperature dependence of the carrier mobility

in CBOA. The carrier mobility decreases in the isotropic state

with decreasing temperature and shows the abrupt decrease at the

isotropic+nematic transition point. Such a step-wise change of

the mobility at the phase transition was also observed in PAAl) as shown in Fig.2 which shows the validity of Walden's rule between Illobility and viscosity, indicating the ion ic transport process. The mobility also decreases in the nematic state and shows step

decrease at the nematic~smectic transition temperature by about

one order. The small mobility value in the smectic state of the

order of 10-6_ 10- 7 cm2/Vsec is explained by the increased

*

CBOA

0 : original sample . : af ter the

appli-cation of 180 V 10-~~---L---~--J

2.9 3.0 3.1 .

xlO- 3 °rl )

Fig.l Temperature dependence of mobility in CBOA. ""' Ul :>

--N

ë

Fig.3 Voltage-current character-is tics in CBOA. xlO- 4 10,..---...,10 -;;; Ol 'M 5 ·5

&

~.~'~

_1? UlOl '( 0 U u'-' 1 L---...L.---.L---...Il .~ 2.3 2.4 2.5 2.6 > °K-l ) xlO- 3 Fig.2 Temperature dependence

of mobility. xlO- 7 5 4 CBOA 50p Temp. 52°C 50 100 applied voltage (V) viscosity in the smectic state, indicating also the ionic transport process. The mobility in the smectic state is strongly dependent on the applied voltage as shown in Fig.l. The sample on which the high voltage (90-180 V) has once applied in the smectic state showsthe lower mobility values by about factor 3 compared with the

·original sample(dotted line). The activation energy of the mobi-lity is smaller in the high mobimobi-lity state (HM state) than the low mobility state (LM state). The optical properties of the HM state and the LM state are fairly different. In the HM state, the transmission of the light polarized parallel to the rubbed direc-tion of the electrode plate is larger than the light polarized perpendicularly. However, such a polarization dependence of the transmission was not observed in the LM state. The HM state can be easily transformed into LM state by the application of high voltage. However, the transformation from the LM.state to the HM state is quite difficult and the LM state is sustained for long time af ter the removal of the applied field, indicating the memory

effect. The heating of the sample upto the nematic state is the only method to re cover the initial HM state. The existence of the two states of the LM and HM states can explain satisfactorily the apparent negative resistance of the current voltage character-is tics shown in Fig.3.

These results can be explained by the following speculation of the molecular alignment. The independence of the transmission on polarization indicates that all molecules aligned with their axes perpendicular to the glass surface under the LM state. On the contrary, in the HM state the molecule axis near the surface is

forces to align parallel to the rubbed direction of the glass plate. In the middle of the sample cell, however, the molecule axis is not completely aligned parallel to the electrode surface, though it liesin the same plane with the direct ion of rubbing. Under these conditions, the sample is effectively uniform for the light pQlarized perpendicularly to the rubbed direction. For the light polarized to the rubbed direction, however, the reflective index is nonuniform in the middle part of the sample, resulting in the light scat tering. As CBOA is p-type material, application of the sufficient high voltage induces the reorientation of the mole-cule axis from the perpendicular to the parallel alignment. along the field direction, resulting in the change from HM to LM state. The observed larger dielectric constant in the LM state compared with HM state is consistent with this argument. Because of high viscosity in the smectic phase compared with the nematic state, reorientation requires fairly high voltage and the transformed state remains for a long time af ter the removal of the applied field. The observed larger activation energy of mobility in the LM state indicates the larger potential barrier to.slip out from the smectic layer compared with that for the movement in the smectic layer. These resultsseem to support the assumption of the ani-sotropic conduction in the smectic made by E.F. Carr. 2) Magnetic field dependence of the mobility in the smectic state also supports the above discussion.

Effects of phase transition between various smectic states on the mobility was also studied. Field dependence of the mobility in PAA (nematic) was also studied. The carrier mobility decreases with increasing applied voltage and showed the minimum value of about (2/3))lo at about 5 V, where}JD is equal to the mobility value measured at lowest field. Then the mobility increases again leading into saturation. The decrease of mobility was ex-plained by the transformation of the initial parallel molecular alignment to the electric field into the perpendicular alignment by the field effect, because PAA is n-type. Further increase of mobility at high field may be explained by the reorientation of

the axis by the flow alignment and the hydrodynamic flow of liquid. These mobility behavior have correlation with the Williams domain formation and the strong DSM light scattering.

2. Cholesteric Liguid Crystals

Figure 4 shows the temperature dependence of the dark conductivity in CPa. In solid state, conductivity increases remarkably by rais-ing temperature and shows a sharp peak at the solid liquid crys-tal transition point. Small step-wise change of the conductivity

w~s als~ observed at the liquid cr~stal~isotropic liqu~d trans

i-tl.On p01rtt. On the contrary, Shaw ) observed nearly un1form increase of conductivity in all phases with increasing temperature and considered that the results indicate the same conduct ion mech-ani sm in all phases. Shaw et al estimated the carrier mobility from the quadratic dependence of the current on the applied voltage by SCLC analysis to be 1-8 cm2/Vsec. However, we could

not observe such aquadratic dependence. Though there are

sever-al reports on the extremely large carrier mobility of electrons in dielectric liquids~) The life time of these free electron is

( ° r1 )

3.1 x10-3 Fig .. 4 Temperature dependence

of the e1ectrica1 conductivity in cho1e-stery1 pa1mitate. Û 10-5 Ol (IJ :> ;::;- 5

e

' - ' Cho1estery1 Pa1mitate

Fig.5 Temperature dependence of the carrier mobi1ity in cho1estery1 pa1mitate.

considered to be short. According1y the reported large electron mobi1ity in all phases obtained by dc method seems to be unusua1.

We a1so estimated the carrier mobi1ity in the cho1estric 1iquid crysta1s to the order of 10-5 __ 10-7 cm2/Vsec from po1arity rever-sa1 method as shown in Fig.5 which shows step-wise change at the cho1esteric -lp isotropic transition point. The step-wise change

of viscosity at the phase transition5 ) point was a1so reported, indicating the Wa1dens ru1e between the viscosity and our mobi1i-ty. The walden's ru1e was a1so confirmed in other cho1esteric 1iquid crysta1s such as CA etc. The Wa1dens ru1e between mobi-1ity and viscosity, and the magnitude of the mobimobi-1ity of the order of 10-5~ 10-7 cm2/Vsec indicated that the ionic transport is dominant in the cho1esteric 1iquid crysta1s investiyated, being simi1ar to the nematic and smectic 1iquid crysta1s~

References

1) K. Yoshino, S. Hisamitsu and Y. Inuishi; J. Phys. Soc. Japan, 32 (1972) 867.

2) ~P. Carr; Phys Rev. Letters, 24 (1970) 807.

3) D.G. Shaw and J.W. Kauffman; Phys. Stat. Sol. (a)

i

(1971) 467, J. Chem. Phys. 54 (1971) 2424.

4) W.F. Schmidt an~A.O. Allen; J. Chem. Phys. 50 (1969) 5037, ibid. 52 (1970) 4788, R.M. Minday, L.D. Schmidt and H.T. Davis; J. Chem. Phys. 50 (1969) 1473, P.T. Tewari and G.R. Freeman; J. Chem. Phys. 49 (1968) 4349.

5) P.S. Porter and J.F. Johnson; J. App1. Phys 34 (1963) 55. 6) K. Yoshino, K. Yamashiro and Y. Inuishi; Japan J. App1. Phys.

ELECTRICAL CONDUCT ION IN NEMATIC LIQUID CRYSTALS

N. Félici: B. Gosse*, J.P. Gosse*

Devices using nematic liquid crystals (LC) effects such as dynamic scat tering (DSM) and storage require the passage of a DC to opera

-te. Applying a voltage (~20V) across a LC film (~ sO~m) creates a strong turbulence forming scat tering centres ; this turbulence is generally thought to be caused by ion injection as suggested by Félici (1), but Helfrich's theory (2) which explains the electro-hydrodynamic instabilities under AC drive, has not been completely ruled out under DC drive. On the other hand, one the major con-cerns in LC device technology is the problem of operating life (3). Under DC exitation, a loss of performance occurs, the active

scattering area of the cell diminishes and other LC characteris-tics as color, response time, transition temperature are altered. When carefully purified, these mildly polar compounds have a con

-ductivity in the range 10-11 to 10-9 (~cm)-l due to dissociated impurities. Their electrical conduction is very similar to that of other polar liquids and our electrochemical approach (4) is no doubt relevant.

Materials

We investigated various typical LCs which are relatively simple

H

organic compounds : Schiff bases X~C N~Y for example p-methoxybenzilidene p-butylaniline (MBBA) Y C4H9 (5) ; tolanes: X~C biphenylnitriles X~ C Experimental facts ~ C~Y (6) ; and N.

x

The investigated LCs are chemically stabie against hydrolysis, except MBBA which forms p-butylaniline (pBA) and p-anisaldehyde

(pAde) according to the reaction : MBBA + H20 ~ pBA + pAde As concerns oxidation and reduction, their redox potentials in acetonitrile on a platinum disk elektrode are given in table I, vs. the Ag/Ag+ reference electrode.

~

formed by MBBA Biphenylnitrile

MBBA _hydrolysis Tolane

Alkyl Alkoxy

~action pBA pAde

1E1 /2 oxid. 1,1 0,5

-

1 1,6 1,3

(V)

1E1/2 red.

(V)

-2,4 - -2,3 - 2,8 - 2,4 - 2,4

• Laboratory for Electrostatics, Centre National de la Recherche Scientifique, B.P. 166, Centre de Tri,

o

lt is seen that the potential for reduction is quite similar for the various compounds ; at the opposite, the potential for oxida-tion is quite low for pBA and rather high for biphenylnitriles. The first step of LC reduction is always LC + e ~ LC':' ; the oxida-tion of the anion radical LC~ gives LC again. LC:', which has been detected by EPR for all the investigated materiaIs, is chemically reactive with protons, water and easily reduced compounds. lts life-time is about lOs for MBBA and pAde, and sti11 longer for bi-phenylnitriles.

Oxidation of LC gives protons (with a faradic efficiency of 1) and neutral by-products more difficult to oxidize than the primitive LC, except in the case of pBA (from MBBA). We have met only once a different oxidation process : with heptoxybiphenylnitriles (HOBN) which form HOBN+.

The chemical evolution of LCs under DC excitation was studied in air-tight cells (gap = 2mm, E = 2S KVcm- l ). The chromatograms of a MBBA sample which initially contains 300 ppm of water show that pBA and pAde are formed at the beginning of the experiment (fig.I). Af ter about 300 hours, partial destruction of; pBA has occured with

---200

...

,

""

400 2.

t:

600 800

Fig.l : lmpurity percentages (Ia: pAde, ~pBA) _{in MBBA, 300 ppm H20, during} a long application of a DC voltage

faradic efficiency of 0.25 to 0.5, and the current density under constant vol-tage starts growing with time. Af ter 36 days, the transition temperature has decreased from 46,SoC to 44,3°C, and the colour has changed from yellow to light brown. Later on, a partial disappearance of pAde is noticed together with an increasing

forma-tion of new compounds. By then, the sample has tur-ned black and its transi-tion temperature is low (36°C).

Tolanes and biphenylnitri-les are altered more slowly than MBBA ; gas chromatography detects a few new compounds in small amounts, the current-time curves only show a slight increase in conductivity (fig.2) ; their transition temperature decreases by about lOC af ter 900 hours, the correspon-ding decrease in MBBA is SoC.

Discussion

Two of the basic problems in LC are : the operating iife and the contribution to current of electrolytic dissociation.

An acceptable operating time for a LC device utilizing DSM is about 104 hours, with a current density of 1 ~A cm-2 (7) ; assuming an electrode area of 1 cm2 and a gap of 10 ~m, the amount of LC in

the cell is 10-3 cm3 or roughly 5.10- 6 mole. The quantity of elec-tricity passed through the cell is about 70 F/mole. Thus, the amount of material deèomposed or altered by any mechanism whatsoever, must